The chemical formula of levonorgestrel USP is 18, 19-Dinorpregn-4-en-20-yn-3-one, 13-ethyl-17-hydroxy-, (17a)-, (-)-, and the chemical formula of ethinyl estradiol USP is 19-Norpregna-1,3,5(10)-trien-20-yne-3,17-diol, (17a)-. The structural formulas are as follows: